This invention relates generally to wireless CDMA communication systems. More particularly, it relates to techniques for inexpensively and efficiently extracting coherent spatial signal information corresponding to multiple user transmitters in a CDMA cellular telephone network.
For various purposes it is sometimes considered useful and/or necessary in radio communications to obtain spatial signal information, i.e. to use an antenna array to coherently receive signals originating from a remote transmitter. Because the signals received at the antenna array retain relative phase information, the signals contain useful spatial information. For example, beamforming techniques are based on the use of coherent spatial signal information to reduce interference and increase system capacity. In another example, Hilsenrath et al. in U.S. patent application Ser. No. 08/780,565 (which is not admitted to be prior art by its mention in this background discussion) disclose a unique method for location finding that is based on the measurement of spatial signal information. Because the spatial signal information is highly correlated with location, this technique performs especially well in multipath environments, and requires only a single base station.
Because code division multiple access (CDMA) systems are becoming more widespread, there is a particular need for measuring spatial signal information in wireless communication systems based on CDMA. As is well known, CDMA is a spread spectrum wireless digital communication technique that enjoys some reduction in the effects of multipath. In contrast to earlier FDMA techniques that assign users to narrow frequency channels in the band, CDMA does not limit individual users to narrow frequency channels but spreads them all throughout the frequency spectrum of the entire band. Signals sharing the band are distinguished by assigning them different pseudonoise (PN) digital code sequences. The well-known correlation receiver uses this known signal structure to decompose multipath parts, provided they are separated in time by at least one chip. The different parts can then be recombined using a RAKE receiver to improve signal strength.
Although presently existing correlation receivers help reduce the effects of multipath on signal fading, they do not provide any coherent spatial signal information. Measuring spatial signal information in CDMA systems, therefore, requires that existing receivers be redesigned and remanufactured. This very expensive and long-term approach, however, does not address the existing need for providing such spatial signal information.
Accordingly, it is a primary object of the present invention to provide a method for extracting coherent spatial signal information using existing CDMA receivers. It is another object of the invention to provide a CDMA receiver that inexpensively and efficiently extracts spatial signal information corresponding to a collection of user transmitters. These and other advantages will become apparent from the following description and accompanying drawings.
In one aspect of the present invention, a CDMA receiver is provided for measuring spatial signal information corresponding to several user transmitters. The receiver comprises a bank of coherent receivers coupled to an antenna array, a bank of signal buffers for recording samples of the received signal data, and a bank of I/Q despreaders for despreading selected signal samples. The receiver also comprises a buffer control circuit for selecting active power control groups in the recorded signal samples for despreading, and a despreader control circuit for selecting PN despreading sequences required to despread the selected power groups. Selected data samples from the signal buffers are fed into the despreaders. The output of the despreaders is a set of spatially correlated I/Q data streams divided into temporal frames, where each frame contains active power control groups from different user transmitters.